The present invention relates to quick-change chucks in general, and more particularly to quick-change chucks for thread-cutters.
Conventional quick-change chucks comprise an extension-compensating or adjusting device which adjusts the chuck in the axial direction during the axial compression and the axial tension. The compression-compensating device and the tension compensating device can be combined in one device. This device is positioned in the lower region of the chuck head between the chuck head itself and the casing receiving a quick-exchange insert which holds the end of the tool. The chuck head is a one-piece structural component with the receiving member, which has a shaft which merges into a chuck head-cylindrical wall of a greater diameter. The cylindrical wall of the chuck head has an internal opening in which the above-mentioned casing is received. Both pressure-loaded springs for the compression and tension compensation are positioned within the bore provided in the receiving member and are supported against the element rigidly connected with the casing. This conventional quick-change chuck makes an axial longitudinal adjustment or compensation possible either under the compression loading or under the tension loading.
Individual situations, in which such an axial compensation is required, are known. The disadvantage of the aforedescribed quick-change chuck is that the maximally possible longitudinal compensation is relatively small as compared to the length of the chuck. Efforts have been made to compensate substantially large axial lengths of the chucks by means of a length-compensation device. Such a device is desired, for example when a cored hole is unintentionally not drilled and the thread-cutting process should continue without, however permitting tap drills to be cracked and the workpiece and the chuck to be damaged. Efforts have been made in such cases to permit the machine cycle to be continued, and the axial length compensation, here during the pressure-loading, must be carried out over the entire machine stroke. Such a situation then becomes similar to the situation when during the cutting of a thread in a predrilled hole the tap drill is uniformly clampled from the start and the torque coupling in the quick-change insert is released so that the torque is not translated to the tap drill. Then the following stroke of the machine spindle is compensated practically over the entire stroke path via the axial compensation in the chuck. The same situation occurs in the reverse rotation direction, for example when the tap drill, upon reaching the bottom of the hole and at the end of the work stroke must be there clamped. When the torque coupling in the quick-change insert is released an undisturbed withdrawal of the machine-side components can be obtained whereby the path difference is compensated via the tension-loaded length-compensation device. Upon passing over the entire compensating length the quick-change insert positioned in the casing of the chuck head becomes axially uncoupled so that this insert can remain, together with the tap drill, on the workpiece, whereas the machine spindle together with the quick-change chuck, will be moved away from the workpiece. A greater axial length-compensation is then also necessary. Inasmuch as the chuck head is formed as a single piece with the spindle-receiving member a complete quick-change chuck must be manufactured for each type of the machine spindle. This leads to high costs of the quick-change chucks. At least one torque-transmitting ball of the length-compensating device for compensating a compression and tension is held in one slot within the cylindrical wall of the chuck head, this slot extending over the entire length of that cylindrical wall. This means that the torque-transmitting ball is supported against the wall of the slot in the cylindrical wall in a point-like manner. Therefore the balls for translating torque in such arrangements are soon damaged. The base of the slot is also worn out. Thus the whole quick change chuck becomes worn out. Furthermore, the torque-transmitting balls must be supported in the chuck with a somewhat radial play. Upon the action of great radial forces there is a danger that the torque-transmitting balls could be displaced and clamped within the chuck, which is also possible due to the above mentioned point-like contact of the balls with the base of the slot. The whole length-compensating device then becomes jammed, or at least it is operated with difficulties. The wearing elements of the quick-change chuck are positioned between the casing and the chuck head. In the case of wear of any of the elements the whole quick-change chuck must be exchanged for a new one.